This invention relates generally to flexible drive devices and more particularly to flexible drive, recanalizing catheters for intravascular surgery.
In U.S. Pat. No. 4,445,509 (Auth) there is disclosed a catheter apparatus for recanalizing (opening) a passageway, e.g., and artery, which has been occluded by intra-arterial deposits of atherosclerotic plaque. That recanalization catheter includes a multi-fluted, rotary cutting head mounted at the distal end of the catheter and arranged to be rotated by a flexible drive shaft extending down the center of the catheter. The drive shaft is arranged to be rotated within the catheter by an electric motor coupled to the proximal end thereof. The drive shaft is disclosed as being a steel helical coil of approximately 0.05 inch (1.3 mm) diameter. Such a coil is stated in the patent to be successful in transmitting high rotational speed (greater than 25,000 rpm) in a controlled fashion and with mechanical security.
In order for a recanalizing catheter to have wide applicability of use in intravascular surgery, its length should be sufficiently large, e.g., 2 to 3 feet or more, while its outside diameter, at least adjacent the working end, is sufficiently small, e.g., 3-4 mm. Moreover the catheter should be able to bend through a minimum diameter radius of curvature of 3 inches or less, in order to reach small, remotely located restrictions, e.g., occlusions.
As will be appreciated by those skilled in the art the torsional shear stress produced on a flexible drive shaft (e.g., a wire) will differ for different composition wires, e.g., approximately 150,000 psi for steel wires, 70,000 psi for beryllium-copper wires. If the radius of curvature through which the drive shaft must bend is very small, e.g., less than 3 inches, high bending stresses will be induced therein. In order to reduce bending strain the diameter of the flexible drive shaft or wire must be made very small, e.g., 0.02 or less inches. If the restriction opening tool is to be operated at a high rate of speed, e.g., greater than 20,000 rpm, in order to provide sufficient power at low torque, the deleterious dynamic effects of critical whirl and friction caused by high side loads on the bearing surfaces supporting the drive wire must be overcome or minimized while the positional neutrality (centering) of the drive shaft is maintained in order to insure that proper operation ensues. The flexible drive systems of the prior art as set forth above appear to leave much to be desired from the standpoint of effectiveness and efficiency of operation in applications involving high speed, small diameter, and small radius of curvature.
In my copending U.S. patent application Ser. No. 06/701,063, filed on Feb. 13, 1985, entitled Shaft Driven, Flexible Intravascular Recanalization Catheter, which application is assigned to the same assignee as this invention, and whose disclosure is incorporated by reference herein there is disclosed and claimed a flexible drive assembly for use with recanalization catheters which overcomes the above noted disadvantages of the prior art. To that end the drive assembly as set forth in that application is a flexible assembly which is arranged to be disposed within a very small diameter, elongated, flexible tubular member having a distal end at which a tool, e.g., a cutter, is mounted for high speed rotation. The drive assembly includes a flexible drive shaft, e.g., a continuous length solid bodied or tubular wire or group of wires, which is mounted within plural spaced bearings to enable it to be rotated at a very high rate of speed from a remotely located motor. The bearings each basically comprise ball-like members having a central opening through which the drive shaft extends. The ball-like bearing members are spaced apart by plural spacer elements at a distance no greater than one-half the wave length of the standing wave which would naturally result from the rotation of a correspondingly sized but unsupported wire at the rotational speed. Each spacer element basically comprises an elongated tubular member having a pair of flared ends. Each of the ball-like bearing members is located between the trailing flared end of one spacer member and the leading free end of the next succeeding, proximally located spacer. Thus, the bearings and associated spacers serve to keep the drive shaft centered even as the catheter is bent through a sharp radius of curvature, while also preventing the shaft from going into critical whirl. A cooling fluid is preferably provided throughout the interior of the tubular member to cool and lubricate the bearings and driveshaft.
While the invention disclosed and claimed in my above noted copending application is suitable for its intended purposes, it is nevertheless somewhat complex in construction. Hence the need exists for a flexible drive shaft having a simple and inexpensive bearing system for supporting an elongated rotary drive element at a central or neutral position while precluding it from going into critical whirl.
Various U.S. patents relate to powered drive shafts, particularly flexible drive shafts and including spirally wound bearings. For example, U.S. Pat. No. 3,180,625 (Wyzenbeek) discloses a flexible shaft having an outer casing enclosing a rotating core and providing a mouth for a spirally wound bearing. The bearing includes a rib for supporting the rotating core. U.S. Pat. No. 4,112, 708 (Fukuda) discloses a flexible drive cable having a rotary flexible core member formed of plural strands. The core is supported in a tubular elongated liner member. In some embodiments the liner member is in the form of spirally disposed surfaces to provide plural lubrication grooves therebetween. U.S. Pat. No. 1,785,345 (Hasemann) discloses an elongated flexible shaft such as used with an automobile speedometer located within a casing forming a bearing for the shaft. The cable is supported by a strip formed in a spiral and made of a wear-resisting hard metal. U.S. Pat. No. 2,821,092 (Cordoro et al.) discloses a rotary drive cable to which is fixedly secured a spiral element. The element serves to hold and position the central core within a liner and is formed of Teflon.
While the devices in all of the foregoing patents may be suitable for their intended purposes, none appears suitable for high speed operation in very confined, small radius of curvature applications, where drive element positional neutrality and resistance to critical whirl is of major importance, such as exists in an intravascular catheter device.
Other prior art devices utilizing flexible drive shafts for conveying rotary power to a working head or tool are disclosed in the following U.S. Pat. Nos.: 1,481,078 (Albertson), 1,636,038 (Bolozsky et al.), 2,570,335 (Fitch), 2,761,297 (Buchsteiner et al) and 4,424,045 (Kulischenko et al).